CVE-2025-67855: Moodle Reflected XSS Vulnerability

CVE-2025-67855 Overview

A reflected Cross-Site Scripting (XSS) vulnerability has been identified in Moodle's policy tool return URL functionality. This security flaw allows remote attackers to inject malicious scripts through specially crafted links by exploiting insufficient sanitization of URL parameters. When a victim clicks on a malicious link, the attacker-controlled script executes within the user's browser context, potentially leading to information disclosure or arbitrary client-side script execution.

Critical Impact

Successful exploitation enables attackers to execute arbitrary JavaScript in victims' browsers, potentially stealing session cookies, credentials, or performing actions on behalf of authenticated users.

Affected Products

• Moodle (specific versions not disclosed in CVE data)

Discovery Timeline

• 2026-02-03 - CVE-2025-67855 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2025-67855

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw exists in Moodle's policy tool where the return URL parameter is not properly sanitized before being reflected back in the HTTP response.

When users interact with Moodle's policy acceptance workflow, the application uses a return URL parameter to redirect users after completing policy-related actions. The insufficient validation of this parameter allows attackers to craft URLs containing malicious JavaScript payloads that execute when the page renders in the victim's browser.

The attack requires user interaction—specifically, the victim must click on an attacker-crafted link. Once triggered, the malicious script runs with the same privileges as the authenticated user, enabling various attack scenarios including session hijacking, credential theft, and unauthorized actions within the Moodle platform.

Root Cause

The root cause of this vulnerability is insufficient input sanitization in the policy tool's URL parameter handling. The application fails to properly encode or validate the return URL parameter before including it in the rendered HTML response. This allows special characters and JavaScript syntax to pass through unfiltered, enabling script injection.

Proper implementation should include output encoding appropriate to the context (HTML entity encoding, JavaScript escaping, or URL encoding) and input validation to ensure URL parameters conform to expected formats.

Attack Vector

The attack is network-based and requires user interaction to succeed. An attacker constructs a malicious URL containing JavaScript payload in the return URL parameter of Moodle's policy tool. This URL is then distributed to potential victims through phishing emails, social engineering, forum posts, or other means.

When a victim clicks the link while authenticated to the Moodle instance, the malicious script executes in their browser session. The attacker can leverage this to steal session tokens, capture keystrokes, redirect users to phishing pages, or perform actions within Moodle on behalf of the victim.

A typical attack flow involves embedding JavaScript payloads within URL parameters that bypass insufficient sanitization filters, then tricking users into visiting the crafted URL.

Detection Methods for CVE-2025-67855

Indicators of Compromise

• Unusual URL patterns in web server logs containing encoded JavaScript in return or redirect parameters targeting the policy tool
• Unexpected outbound connections from user browsers to external domains during Moodle sessions
• User reports of unexpected behavior or redirects when accepting policies
• Web application firewall (WAF) alerts for XSS patterns in Moodle URL parameters

Detection Strategies

• Configure web application firewalls to detect and block common XSS payloads in URL parameters
• Implement Content Security Policy (CSP) headers to restrict script execution sources and report violations
• Monitor server access logs for suspicious URL patterns containing encoded script tags or event handlers
• Deploy browser-based security tools that detect and prevent DOM manipulation attacks

Monitoring Recommendations

• Enable detailed logging for Moodle's policy tool endpoints and review for anomalous parameter values
• Set up alerting for CSP violation reports that may indicate XSS exploitation attempts
• Monitor authentication events following policy tool interactions for signs of session compromise
• Implement real-time log analysis to detect URL parameters containing suspicious encoding patterns

How to Mitigate CVE-2025-67855

Immediate Actions Required

• Apply the latest Moodle security patches addressing this vulnerability when available
• Implement Content Security Policy headers to restrict inline script execution
• Configure web application firewall rules to block requests with XSS patterns in URL parameters
• Educate users about the risks of clicking suspicious links, especially those targeting Moodle policy endpoints

Patch Information

Refer to the Red Hat CVE-2025-67855 advisory and Red Hat Bug Report #2423861 for the latest patch information and updates from Moodle maintainers. Organizations should prioritize applying vendor patches as they become available.

Workarounds

• Implement strict Content Security Policy headers that disallow inline scripts and restrict script sources to trusted domains
• Deploy a web application firewall configured with XSS detection rules to filter malicious URL parameters
• Consider temporarily disabling or restricting access to the policy tool if it is not critical to operations
• Use browser extensions or endpoint security solutions that provide XSS protection for end users

# Example Content Security Policy header configuration for Apache
# Add to .htaccess or Apache configuration
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'self';"